Compression molding two or more polytetrafluoroethylene resin layers to form a pressure pad

ABSTRACT

Methods of compression molding powdered polytetrafluoroethylene resins to produce composite articles of manufacture having at least two distinct layers of two types of PTFE resin are disclosed. A first layer of resin, preferably including a filler, is placed in a compression mold and partially compressed at a relatively low pressure. A second layer of resin, preferably virgin resin, is then added to the mold and both layers are simultaneously fully compressed. After compression, the resulting composite preform is preferably sintered and coined to create a finished PTFE material or product. In a preferred embodiment, the partial compression step is performed at pressures of less than 500 psi. It has been found that modified PTFE resins provide improved resistance to cold flow and also provide better binding between filled and unfilled (virgin) layers and thus are used in certain preferred embodiments.

This is a division of application Ser. No. 08/079,163, filed Jun. 18,1993, now U.S. Pat. No. 5,399,307.

The present invention relates to compression molding two or more layersof powdered polytetrafluoroethylene (PTFE) resin and composite productsmade by such compression molding. More particularly, the presentinvention relates to forming composite articles by compression moldingwherein the finished article exhibits two distinct layers or areas ofmaterial, preferably one of filled PTFE and one of virgin PTFE. Thepresent invention also discloses the use of modified virgin PTFE resinsto enhance the integrity of such composite structures.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) is a versatile and useful material. Inthe form of a powdered resin, PTFE can be molded in sheets and othershapes, or directly into a finished part or product. Several types ofmolding processes useful with PTFE have been developed. Compressionmolding involves placing a layer of PTFE resin under sufficient pressureto create a preform that can be sintered into a finished shape, which isusually then coined. Isostatic molding involves filling the void betweena rigid molding surface and a flexible bladder with PTFE resin. Thebladder is then pressurized, and squeezes the resin between the bladderand the molding surface. The bladder is then removed and the resin issintered. Isostatic molding is expensive and cumbersome, but iswell-suited for forming complex and intricate shapes such as valvelinings. In fact, the body of the valve itself can be used as themolding surface, thus permitting the valve lining to be formed in situwithout requiring secondary shaping or cutting operations. However, formany applications, compression molding remains the simplest and mosteconomic method of producing PTFE articles.

Sheets, bars or other shapes of compression molded PTFE are used tocreate many useful articles that take advantage of its low coefficientof friction and chemically impervious nature. However, the structuralcharacteristics of a shape formed from virgin PTFE are less than idealfor many situations. It is known, for example, that virgin PTFE exhibitsa large degree of cold flow (i.e., "creep") when exposed to continuousloads at elevated temperatures. For example, at 500 psi and 73° F.,virgin PTFE will cold flow from an initial deformation of 0.9% to about1.8% in 10,000 hours. At 212° F., an initial deformation of 3.5% wouldincrease to approximately 7% in the same time period. It is also knownthat PTFE exhibits deformation due to compressive stress. At stresslevels of 500 psi and at a temperature 73° F., this strain is almostnegligible, while at 300° F. the strain at this loading is about 5%. Asimilar strain level, 5%, is exhibited at 73° F. when the compressivestress is raised to about 1,900 psi. Thus, in applications where thePTFE component is under a compressive load and/or exposed to elevatedtemperatures, cold flow and deformation due to compressive stress areimportant considerations.

Nevertheless, it is known that the problems of cold flow can in someinstances be overcome by adding fillers to virgin PTFE resins. The useof filler materials is well known in the art of forming shapes fromPTFE. See "Selecting the Right `Teflon` TFE Compound For BestPerformance," Journal of "Teflon" Volume 13, No. 2 (1972). Examples offiller materials include glass fibers, carbon, graphite, bronze andmolybendum disulfide (MoS₂). The modification of the performance of PTFEresins by the use of fillers affects certain mechanical properties andpermits resin filler compositions to be tailored to the electrical andmechanical requirements of the application. In general, PTFE resins canbe compounded with fillers to increase strength characteristics such asstiffness and resistance to cold flow, to increase thermal conductivity,and to increase hardness and resistance to wear. However, it is alsoknown in the art that any filler raises the low coefficient of frictionexhibited by parts formed from virgin PTFE resins and that the increasesin hardness and resistance to wear result in an article that abrades orwears cooperating surfaces.

Modified PTFE resins are currently available that provide productsexhibiting improved resistance to cold flow without sacrificing the lowcoefficient of friction of virgin PTFE. Additionally, products made fromthese modified resins have been found to exhibit better "weldability" tothemselves and other PTFE products when used in compression molding.Examples of such modified PTFE resins are TFM1600 and TFM1700 sold byHoechst, and TG70J and TG170J sold by dupont. One example of such aresin is disclosed in U.S. Pat. No. 4,408,007--Kuhls et al., which isincorporated herein by reference. In some instances the use of suchresins is economically impractical since they cost between about 50% and180% more than standard virgin PTFE.

Therefore, there remains a long felt, yet unsolved need to providecomponents formed from PTFE that retain the properties of virgin PTFEsuch as a low coefficient of friction on sliding surfaces, that alsoexhibit acceptable resistance to cold flow or have other improvedproperties usually provided by filled and/or modified resins, and thatprovide this combination of properties in an economically viable form.

Composite PTFE articles are known that are comprised of different typesof PTFE. U.S. Pat. No. 5,032,335--Wilson discloses sealing elementssliced from a cylindrical billet comprised of two types of PTFE that aresintered together. The cylindrical billet is formed by arranging twoseparately formed, unsintered tubular billets concentrically within oneanother then adding pressure to the combined part and sintering theresulting composite billet. One of the unsintered tubular billets iscomprised of filled PTFE while the other uses virgin resin. Theseparately formed, unsintered billets are compacted using a pressurebetween 250 to 2,500 psi. After assembly, an axial compressive pressureof between 2,000 to 25,000 psi is applied to fully compress unsinteredbillets into a preform, which is then sintered at 650° F. to 750° F. fortwo to 48 hours. A "sandwich" PTFE structure consisting of a layer ofunfilled PTFE between two filled layers is disclosed in U.S. Pat. Nos.4,961,891 and 4,900,629, both to Pitolaj. The disclosed compositestructure is said to be highly compressible and is made by fusing sheetsof PTFE together using calendar rolls. The resulting laminate is thensintered at 650° F. A multi-layer PTFE seal is also disclosed in U.S.Pat. No. 4,147,824--Dettmann et al. The disclosed seal comprises anouter layer and a porous layer made by washing a filler material fromthe PTFE. The first layer is fully compressed with a pressure of 300 bar(4,410 psi) and a second, filled layer is added and compressed with thesame pressure. The resulting preform is then sintered and the fillermaterial washed out, leaving a porous layer.

Isostatic molding can also be used to create composite structures fromhomogeneous preforms of different types of PTFE to achieve a unitarynon-homogeneous structure, as shown in U.S. Pat. Nos. 4,267,237 and4,102,966 both to Duperray et al. The preforms must be compressed by apressure of at least 10 bar (147 psi) since compression using lesserpressures will create preforms that are too brittle to be handled. Thepreforms are placed in an isostatic mold and further compressed to fusethem together or fuse them with a layer of powdered resin usingisostatic pressures between 100 and 1,000 bar (1,470 and 14,700 psi).

However, it has been found that processes by which fully sintered sheetsof two types of PTFE are laminated together are likely to delaminate,particularly under severe conditions of temperature and pressure.Conversely forming a preform from two layers of powdered resin that aresimultaneously compressed will result in undesirable mixing andcontamination of the separate "layers" of different types of PTFE.

One example of an application where the current state of the art failsto provide an adequate material is for the pressure pads used to assliding surfaces for time belts that compress sheets of particleboard aspart of the manufacturing process performed by the Bison Hydrodyn press,manufactured by Bison Werke, Springe, Germany. The construction andoperation of this press and similar types of manufacturing equipment iswell known to those of ordinary skill and is shown, for example, in U.S.Pat. No. 4,850,848--Greten et al., which is incorporated herein byreference. As shown in FIG. 1, the Bison press and similar machinesapply heat and pressure simultaneously to the components of the finishedparticleboard throughout the length of the machine. The heat is appliedby hot lubricant on to the back side of stainless steel bands. Thelubricant is pumped through PTFE pressure pads and provides a layer oflubricant between the pads and the bands, in addition to acting as aheat transfer medium. The PTFE pressure pads are attached to the platensthat transfer vertical compression force provided by hydrauliccylinders. As seen In FIGS. 2-3, the pressure pads 52 are held inremovable carrier trays 50 that form part of the press platens. Thecarrier trays 50 lave angled slots 54 to retain the lateral edges of thePTFE pads 52 while retaining plates 56 are affixed to the perimeter ofthe carrier trays 50 to secure the pressure pads 52 in place. Thelubricating fluid flows through passages 53 in the pads 52 andlubricates the bands.

In practice, it has been discovered that the pressure pads 52 in theabove-described system fail in service by a mode referred to as"smearing." Smearing is the migration and thinning of the pressure padswithout any wear, in other words, there is no appreciable weightdifference between a new pad and a failed pad. Theoretically, thestainless steel belts ride on a layer of oil, the heating oil, that ispumped through the pads between the back surface of the stainless steelbelts and the pads. Although the layer of oil is relied upon to preventthe pad from scraping and wearing against the stainless steel belts, inuse, the hot oil system sometimes fails and the low friction coefficientof friction between the PTFE and the stainless steel belts is helpful.Moreover, the softer PTFE pads will wear before the stainless steelbelts. For these reasons, PTFE is the preferred pressure pad material.However, because the operating temperatures (360°-390° F.) and pressures(300-500 psi) are relatively high, the PTFE exhibits a significantdegree of cold flow, ultimately leading to the "smeared" failure modedescribed above.

Therefore, in this type of equipment and many others, it would bedesirable to provide a PTFE component such as the pressure pads that hasan exposed sliding surface with a low coefficient of friction, yetresists cold flow at elevated temperatures and pressures. It be furtherdesirable to provide such a component on an economically justifiablebasis. It is therefore an object of the present invention to provideimproved methods of compression molding PTFE resin into sheets,structural shapes or component parts. It is generally an object of thepresent invention to provide methods whereby filled and virgin PTFEresins may be compression molded into a unitary non-homogeneouscomposite structure exhibiting two or more distinct layers havingdifferent properties. It is a further object of the invention to permitfilled and virgin PTFE resins to be molded together so that the virginPTFE forms a sliding surface while the remainder of the material iscomprised of a filled resin, wherein the filler is chosen to reduce coldflow. It is also an object of this invention to provide methods ofmanufacture and articles made thereby that make economic use of thevarious types and grades of PTFE resin, including modified PTFE.

SUMMARY OF THE INVENTION

It has now been found, however, that improved products and materials canbe created by compression molding a preform from powdered PTFE resin bydepositing a first layer comprising a first type of PTFE resin andpartially compressing this first layer at a partial compression pressureless than about 500 psi. Next, a second layer preferably comprised of asecond type of PTFE resin is deposited in the mold and the first andsecond layers are fully compressed with sufficient pressure to create apreform. In a preferred embodiment, the first layer is comprised of afilled PTFE resin and the second layer is comprised of virgin PTFEresin, most preferably a modified virgin resin that is resistant to coldflow. In accordance with the certain preferred embodiments of thepresent invention, the partial compression pressure is preferably lessthan 147.0 psi, more preferably less than 14.7 psi., and most preferablyless than 0.4 psi. The methods of the present invention are useful forproducing a variety of products, including sheets and cylinders. Inaddition to the two layer composite structures produced in one preferredembodiment, the methods disclosed herein may also be used to produceother composite structures. For example, the methods described above canalso include the steps of adding one or more further layers of a PTFEresin and partially compressing the layers prior to the step ofcompressing the layer with the preform pressure.

Thus, the present invention discloses articles of manufacture comprisinga first layer of filled PTFE and a second layer of virgin PTFE,manufactured by the process of sintering and coining a preform comprisedof the first and second layers, wherein the preform is manufactured inaccordance with he above-described process. One particular preferredembodiment of such articles is a pressure pad for use in a particleboardmanufacturing apparatus. In this embodiment, an article made inaccordance with the present invention comprises a sliding surface of alayer of modified virgin PTFE and a second layer of filled, modifiedPTFE having significantly better resistance to cold flow than the virginPTFE, such as modified PTFE with a carbon filler material.

In certain embodiments, the present invention provides a superiorproduct by using modified virgin PTFE resins, which have been found toprovide superior binding strength between layers. In any embodiment,however, the present invention uses conventional compression moldingequipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a prior art particleboard press that uses PTFEpressure pads as sliding surfaces urged against moving stainless steelbelts.

FIG. 2 is a top plan view of a carrier plate used in the apparatus shownin FIG. 1 showing the placement of the PTFE pressure plates.

FIG. 3 is a cross-sectional view of the carrier plate of FIG. 2, takenalong lines 3--3 as shown.

FIG. 4 is a side elevation view, in cross-section, representing aportion of a compression molding process illustrating a mold containingfilled resin.

FIG. 5 is a view similar to FIG. 4 illustrating the partial compressionof the filled resin.

FIG. 6 is a view similar to FIG. 4 illustrating the addition of a layerof virgin resin.

FIG. 7 is a view similar to FIG. 4 illustrating the full compression ofthe virgin and filled resin layers into a preform.

FIG. 8 is a graph of deformation vs. temperature for PTFE resins used inpreferred embodiments of the present invention.

FIG. 9 depicts a composite structure made in accordance with the presentinvention wherein a layer of filled PTFE is disposed between two layersof virgin PTFE.

FIG. 10A depicts a composite sheet of filled and virgin PTFE resin.

FIG. 10B illustrates the construction of the composite sheet shown inFIG. 10A.

FIG. 10C shows a side elevation view of the composite sheet shown inFIG. 10A with additional layers of PTFE added in accordance with thepresent invention.

FIG. 11 is a side view, in cross-section of a cylindrical compositemolded partial preform.

FIG. 12 illustrates the deposition of additional layers of virgin PTFEto create a finished preform.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides methods whereby PTFE articles can bemanufactured from two types of PTFE resin and as a result, exhibitdistinct properties on different surfaces and/or improved overallproperties. Referring to FIGS. 4-7, there is shown a broken awaycross-sectional view of a compression mold 200 during the various stepsthat comprise the creation of a preform in accordance with the presentinvention. For purposes of illustration, portions of the compressionmold apparatus such as the bottom pusher used to revise the resin in themold are omitted. These and other elements of compression moldingequipment will be well known to those of skill in the art. As seen inFIG. 4, a rigid lower section of the compression mold 200 is providedand is filled with a first type of PTFE resin 100. In a preferredembodiment, the first resin 100 will include a filler material, mostpreferably carbon. As known to those of ordinary skill in the art, aseries of pushers 210 form part of the rigid compression mold assembly.As seen in FIG. 5, the relatively rigid pushers 210 are lowered againstthe first layer of PTFE resin 100 arid partially compress this layer. Asused herein, the term "partial compression" refers to a process wherebyPTFE resin powder is compressed by a pressure of less than 34 bar (500psi), the pressure being all average pressure determined by dividing theoverall area of the pusher 210 contacting the surface of the resin bythe magnitude of the force that urges the pusher 210 in compression.However, in certain preferred embodiments, it will be desirable toreduce the pressure during partial compression to less than 10 bar (147psi), and in a most preferred embodiment, to less than 1 bar (14.7 psi).Certain useful embodiments, in fact, utilize pressures of less than 0.1bar (1.5 psi)

Referring now to FIG. 6, after the first layer of PTFE resin 100 hasbeen partially compressed, a second layer of PTFE resin 102 is depositedwithin the mold. In a preferred embodiment, this layer is comprised ofvirgin PTFE and is most preferably a modified PTFE resin such as TFM1600and TFM1700 sold by Hoechst, and TG 70J and TG 170J sold by dupont,referenced above. The pusher bars or other form of rigid molding tool210 are then used to fully compress both the first and second PTFElayers 100,102. As used herein the term "fully compress" refers to thepressure normally applied to a particular powdered resin or resins tofully preform the powder prior to subsequent processing steps such assintering and coining. The pressure applied to fully compress thecomposite structures formed with the present invention is normallybetween about 200 to 1,000 bar (2,940 to 14,700 psi). This procedureimparts a mechanically compressed effect to the preform. It should berealized that the properties of the preform will vary with both thepartial compression and full compression pressures, as well as thecomparable shrinkage rates and material grades for the PTFE resins.

It will be understood that the description set forth immediately aboveis meant to generally disclose and illustrate the methods of the presentinvention. An infinite number of variations are possible. For example,more than two layers and more than two types of PTFE resins may be used.Moreover, as also known in the art, compression molding may be used tocreate preforms off an infinite variety of shapes and sizes. Inpreferred embodiments, the present invention takes advantage of thecharacteristics of modified PTFE resins. It has been found that certainmodified resins act as a binding agent between composite layers. In anyembodiment, however, the present invention permits a preform to becreated that contains at least two types of PTFE resin in two distinctlayers and does so in a way that ensures a fully laminated structurewhile maintaining the integrity of the layers.

Although further variations and additional embodiments of the presentinvention will be set forth below, the general principles of the presentinvention are also further illustrated by the following specificexamples.

EXAMPLE I

A compression molded sheet (approximately 5/8 inch thick, 48×48 inches)is formed by first pouring a layer of a filled modified resin (TFM 1700and 20% carbon filler) into the compression mold and partiallypreforming this layer using a very low pressure of about 0.027 bar (0.4psi). Next, a layer of virgin modified PTFE resin (TFM 1600) is added tothe compression mold cavity and this layer and the first layer are fullycompressed at a pressure of about 300-600 bar (4,410-8,820 psi). Theresulting composite preform preferably has a layer of filled PTFE thatis 1/2 inch thick, while the layer of virgin resin is about 1/8 inchthick. The fully compressed preform is then sintered at about 700° F.for about twenty-four hours, and coined in accordance with conventionalpractice.

The resulting non-homogeneous composite sheet may then be cut ormachined into components, such as the particleboard forming machinerypressure pads described above. In this application, the material formedin accordance with the present invention provides a layer of lowfriction, modified PTFE against which time stainless steel belts willslide, while also providing improved resistance to creep, and thusresistance to "smearing" by way of the layer of filled PTFE. Referringto FIG. 8, it can bee seen that at the temperature range in which theparticleboard machinery operates, a composite article made in accordancewith the present invention exhibits markedly better resistance todeformation than virgin PTFE while retaining the desirable lowcoefficient of friction exhibited by virgin PTFE. Moreover, the cost ofthe overall sheet is considerably reduced, since the filler added to themodified virgin PTFE resin is only about 10% of the cost of virginunfilled PTFE.

FIG. 9 depicts a composite structure made in accordance with the presentinvention wherein a layer of filled PTFE 100 is disposed between twolayers of virgin PTFE 102, 103, in accordance with the methods discussedabove. It will be appreciated that any combination of layers of filledand unfilled PTFE resins may be combined using the techniques disclosedherein. Those of ordinary skill will be able to select desired materialproperties and by designing a composite sheet having the appropriateproportions of different resins, the desired properties will beachieved.

Other embodiments of the present invention are illustrated withreference to FIGS. 10A-10C. FIG. 10A depicts a composite sheet 110comprising virgin PTFE sheet 112 with an area 114 of filled PTFE aroundits perimeter. This composite sheet 110 is formed by the process shownin FIG. 10B wherein a compression mold 220 contains the sheet 110 and athin frame 222 separates the virgin sheet 112 and the filled area 114.The frame 222 is removed and the resins are compressed in the mannerexplained above, using a relatively low pressure. FIG. 10C illustrates across-section taken along line C--C in FIG. 10A. As also shown in FIG.10C, additional layers of virgin PTFE 116,118 are spread across thesheet 100 to cover the exposed surface. The resulting compositestructure is then fully compressed into a preform and sintered. Theresulting article is a sheet of PTFE that has a layer of virgin PTFE onall exposed sides (or only certain exposed sides in other embodiments)and an encapsulated "core" of less expensive filled PTFE that alsoexhibits properties that vary from virgin resin, such as improvedresistance to cold flow. Embodiments made in accordance with thisexample are not necessarily rectangular, but may be any shape, forexample, a circle of filled resin within a square frame of virgin resin.Moreover, in some instances, it will be desirable to cut, or otherwisemachine the resulting composite sheet.

The present invention can also be used to create articles that are notsolid. For example, FIG. 11 illustrates a side view, in cross-section ofa cylindrical preform. The preform is preferably comprised of an innerlayer of virgin PTFE 120, an intermediate layer of filled PTFE 122 andan outer layer of virgin PTFE 124. As explained above with reference toFIGS. 10A-10B this preform can be constructed using a cylindrical moldin conjunction with thin separators to permit the required layers ofresin to be deposited. In accordance with he present invention, afterthe internal separators are removed, the preform shown is partiallycompressed using a relatively low pressure. After partial compression,additional layers of PTFE 126, 128, shown in FIG. 11, are disposed uponthe cylindrical ends and the entire preform is fully compressed andsintered. It will be understood that although it is preferred that theouter surfaces of the resulting cylinder are comprised of virgin PTFE,any combination of resins may be used for the disclosed layers. Itshould be further noted that this embodiment, using a cylinder having asmaller height, permits a seal to be formed that has an outer layer ofvirgin PTFE covering all exterior surfaces while encapsulating a secondtype of PTFE that permits the seal to exhibit desired properties such asresilience or resistance to creep that would not be exhibited if thesame seal were formed of virgin PTFE.

In certain embodiments, the present invention takes advantage of theimproved properties of modified PFFE resins. However, it will beunderstood that the methods disclosed herein can be used with virtuallyany type of PTFE resin and any type of filler material to produce usefulproducts in an economical manner. For example, in certain embodiments,the filled resin will be the most expensive component and layers orareas of virgin resin can be used where the properties of the filledresin are not needed. Finally, it will be understood that a benefit ofthe present invention is that the methods disclosed herein can beperformed using conventional compression molding equipment, e.g., molds,presses, punches, ovens, etc. The only additional equipment needed insome embodiments is the thin flexible inserts that separate areas ofdifferent types of resin.

Although certain embodiments of the present invention have been setforth herein with a great degree of particularity, these embodiments aremeant to illustrate the present invention and do not limits its scope.Upon review of this specification, numerous adaptations, modificationsand variations based upon the principles disclosed herein will becomeapparent to those of ordinary skill in the art. Therefore, in order todetermine the true scope of the present invention, reference should bemade to the appended claims.

What is claimed is:
 1. A pressure pad for use in a particleboardmanufacturing apparatus comprising sheet of compression molded PTFEhaving a sliding surface comprised of a layer of virgin PTFE and asecond layer comprised of layer of filled PTFE bonded together bypartially compressing the first layer at a partial compression pressureless than 14.7 psi, and then compression and sintering both layerswithin a mold.
 2. The pressure pad of claim 1, wherein the layers offilled PTFE and virgin PTFE comprise a modified PTFE resin.
 3. Apressure pad for use in a particleboard manufacturing apparatuscomprising sheet of compression molded PTFE having a sliding surfacecomprised of a layer of virgin PTFE and a second layer comprised oflayer of filled PTFE having significantly better resistance to cold flowthan virgin PTFE wherein the layers are sintered together after beingindividually compressed and thereby resist delamination manufactured bythe process of:depositing a first layer of powdered resin comprised ofthe filled PTFE resin into a first portion of a rigid mold assembly;partially compressing the first layer at a partial compression pressureless than 14.7 psi by contacting the first layer with a second portionof a rigid mold assembly; depositing a second layer of powdered resincomprised of virgin PTFE resin; compressing the first and second layerswithin said rigid mold assembly at a preform pressure to create apreform; sintering and coining the preform to create a sheet product;and cutting the pressure pad from the sheet product.
 4. The pressure padof claim 1, wherein the filled PTFE comprises carbon filler material. 5.The pressure pad of claim 4 wherein the carbon filler comprises about 10to 30 percent by weight of the second layer.
 6. The pressure pad ofclaim 4 wherein the carbon filler comprises about 20 percent by weightof the second layer.
 7. The pressure pad of claim 1, wherein thepressure is less than about 1.5 psi.
 8. A pressure pad for compressing asheet of material using a sliding surface, wherein the sliding surfacecomprises a first layer of virgin PTFE and a second layer of PTFE bondedtogether by partially compressing the first layer at a partialcompression pressure less than 14.7 psi, and then compression andsintering both layers within a mold having significantly betterresistance to cold flow than the virgin PTFE.
 9. The pressure pad ofclaim 8, wherein the layers of filled PTFE and virgin PTFE comprise amodified PTFE resin.
 10. The pressure pad of claim 8, manufactured bythe process of:depositing a first layer of powdered resin comprised offilled PTFE resin into a first portion of a rigid mold assembly;partially compressing the first layer at a partial compression pressureless than 14.7 psi by contacting the first layer with a second portionof a rigid mold assembly; depositing a second layer of powdered resincomprised of virgin PTFE resin; compressing the first and second layerswithin said rigid mold assembly at a preform pressure to create apreform; sintering and coining the preform to create a sheet product;and cutting the pressure pad from the sheet product.
 11. The pressurepad of claim 8, wherein the filled PTFE comprises carbon fillermaterial.
 12. The pressure pad of claim 11 wherein the carbon fillercomprises about 10 to 30 percent by weight of the second layer.
 13. Thepressure pad of claim 11 wherein the carbon filler comprises about 20percent by weight of the second layer.
 14. The pressure pad of claim 8,wherein the pressure is less than about 1.5 psi.